flip|cover-switchers: Improve transparent and blurred background

Currently, the flip|cover window switchers use a Plasma Dialog as their
background, which can provide transparency and blur but it's totally
dependent on the plasma style, which may be not be designed for this
purpose, and doesn't look clean with current breeze defaults.

Use instead the same components that are used in the Overview effect,
for consistency and simplicity.

It should also help with efficiency, as the background and blur
effects are done over DesktopBackground special component and
not over the actual desktop.

The code is also prepared with a bool property to not blur the background,
but it doesn't add the user configuration part (yet).